342 INVERTEBRATE MORPHOLOGY. 



organs of prehension. A second portion of the foot lies in 

 the neck region on the ventral surface and has the form of 

 two folds (si), whose edges may be approximated or even fused 

 to form a tube, through which the water contained in the 

 mantle-cavity may be violently expelled, the animal being 

 thereby propelled through the water in a direction of their 

 long dorse-ventral axis. This portion of the foot is termed 

 the funnel and is perhaps equivalent to the epipodium of the 

 Gasteropods. In the majority of forms there projects into 

 the lumen of the funnel a fold (v) arising from the body-wall 

 and termed the valve of the funnel. It is probably homolo- 

 gous with the posterior portion of the foot, the metapodium, 

 of the Gasteropoda, so that all portions of the Gasteropod 

 foot are represented in the Cephalopods, the propodium and 

 mesopodium by the arm bearing portion, the metapodium by 

 the valve just mentioned, and the epipodium by the funnel. 

 In many forms two depressions are to be found on the outer 

 surface of the funnel, which receive two corresponding eleva- 

 tions on the inner surface of the mantle, which thus becomes 

 locked as it were to the funnel during the expulsion of water 

 from the mantle-cavity. 



The mantle (ra) forms a circular fold surrounding the vis- 

 ceral hump, but upon the anterior surface it has usually only 

 a very slight development, while posteriorly there is a wide 

 space, the mantle-cavity (me), between it and the body-wall. 

 Within this space lie the ctenidia (ct), and into it the nephri- 

 dia (ne) and the digestive tract open, the excreta being ex- 

 pelled from it during the expulsion of water from the funnel. 

 The mantle-fold is rather thick as a rule, owing to the pres- 

 ence in it of abundant muscle-fibres, by the contraction of 

 which the mantle-cavity may be considerably reduced in size, 

 and frequently there is a special muscular thickening around 

 the edge of the mantle whereby the mouth of the cavity, 

 widely open during the intaking of water, may be firmly ap- 

 pressed upon the funnel during the expulsive act. In the 

 majority of Cephalopods the integument covering the outer 

 surface of the mantle and of the visceral hump is provided 

 with abundant pigment-cells or chromatophores each of 

 which is provided with a muscular arrangement by which its 



